The prior art includes filters for removing particulates from various dielectric fluids, such as oils and hydraulic fluids, by employing mechanical filtration. When mechanical filtration is employed to filter very fine particles of, for example, about 5 to 10 microns, difficulties may be encountered because these relatively small particles require large bulk and volume filters to avoid significant system pressure drops caused by the necessarily small filter openings.
Alternatives to mechanical filters for fine particle filtration of dielectric fluids include electrostatic filters where the fluid is made to pass through a number of electrodes which are alternately charged with relatively high positive and negative voltages. Porous filter material is placed between the electrodes for trapping the particulates. It appears that the particulates, when subjected to the electric fields created by the application of voltage to the electrodes, are filtered in one of two possible ways. The filter material itself may be charged with the particulates being attracted to the filter material itself. More likely, however, the particulates are charged, either positively or negatively, depending on their composition, and the oppositely charged particles will be attracted to each other and eventually form a floc of larger particulate clumps which will be large enough to be trapped in the filter material. Whenever enough clumps form to effectively block the filter or produce an undesirable pressure drop, the filter must be replaced. While these electrostatic filters have evolved over a number of years, these filters still present a number of problems in terms of ease of fabrication and testing, as well as ease of replacement and safety of replacement, with attendant cost savings.